The invention relates to a method for balancing the wheels of motor vehicles, applicable on wheels having rims consisting of a hub and a circular peripheral part supported by alternate radial struts or spokes and gaps.
Balancing systems normally used in balancing machines are characterised in that unbalance on each correction plane is compensated by applying a weight in a diametrically opposite position to the angular position of the unbalance. This operation, as is well known, comprises the input of data necessary for identifying the correction plane or planes, a calculation of the forces due to unbalance using transducers of forces applied on the shaft the wheel is mounted on, and the processing of the transducer signals with the aim of obtaining the amount and geometrical arrangement on the wheel of the masses or weights needed to correct the unbalance.
According to the type of wheel, the operator can decide whether to perform a correction of the unbalance on a single plane or on more than one.
The correction of the unbalance on one plane only enables correction of only the static unbalance responsible for the "shimmying" of the wheel.
Normally unbalance correction is carried out on two planes, thus compensating both the effects of static and dynamic unbalance.
For practical purposes, in the case of steel rims, the planes are chosen in the proximity of the edges of the rim, so that the correction of the unbalance happens by means of the application of weights provided with a steel spring which easily couples with the edge of the rim thanks to the use of a plierhammer tool.
In the case of light alloy rims, for reasons connected with the shape of the edge or even for aesthetic reasons, normally self-adhesive weights are used.
The majority of balancers include special functions for the gathering of balancing data in cases where the weights to be used are self-adhesive or a combination of these and other weights to be fixed to the alloy rim edge.
These setting functions take account of the fact that the center of gravity of the adhesive weights is not close to the edge of the rim but rather in a position identified using fixed correction parameters with respect to said rim.
The balancing planes setting function is especially utilized, as it enables a correction of the unbalance by means of the application of adhesive weights applied to the inside of the rim, so that once the wheel is mounted they will not be visible.
Some balancers equipped with special automatic functions allow for an exact diameter feeler reading and a correction plane axial position to be inputted in order to calculate correct positioning and rim application of the self-adhesive weights.
All the above-mentioned prior art realizations have in common the fact that, while it is the operator who defines the position of the correction planes, that is, the diameter and the axial position the weights will be applied at, it is the machine programmer who defines the angular position they will be applied at. These realizations cannot be used to carry out the wheel balancing operation on spoked wheels, that is, wheels which have a centering hole and the bolt holes in a central part which is connected to the rim, housing the tire beads, by means of alternated tracts of spokes or struts and gaps.
For these rims, which are usually made of light alloys, it is usually preferable to put the weights on the inside of the rim, so that they are invisible, being hidden behind a spoke or strut.
Traditional balancers do not guarantee that a weight will be positioned behind the spokes of such a wheel; indeed, the fewer struts there are on the wheel, the greater the probability that the weights will have to be visible, even though they are on the inside of the rim.
In an attempt to overcome this problem, since the 1980s the balancing industry has been using a method which vectorially decomposes the unbalance in two predetermined directions.
This method enables the weight of two weights to be calculated to be applied in two predetermined angular positions (and not coinciding with the position of the detected unbalance), with those two predetermined positions being decided on the basis of where there is material to support the weight.
For example, in the case of axial ventilator balancing, the possible directions correspond with the axes of the rotor blades. This situation is demonstrated, for example, in U.S. Pat. No. 4,357,832, where, in column 8, line 58, the following is stated: "In certain cases, there may be only certain tapped holes equally spaced around the rotor into which correction weights can be threaded. Similarly, in the case of a bladed turbine, for example, the correction weight will have to be added to one of the blades since it cannot be supported in the space between them. The correction vector, however, may be located in-between blades or in-between tapped holes. In this case, it is desirable to resolve this single vector into two vectors which intersect the blades or the tapped holes."
In European Patent no. 95196735.4 a similar unbalance decomposition method is described, in the which the angular positions of the radially-directed parts of the disc of the wheel are memorized and the correction weight is discomposed into two weights, to be applied at the continuous part of the wheel.
All of the above-described methods for vectorial decomposition of unbalance require a prior identification of the zones which can be used as directrices of the vector unbalance or the identification and memorization of the angular position of the material interruptions and/or the continuous radially-directed parts of the wheel disc.
In the very frequent case in which there is a much higher percentage of continuous surface in relation to empty surface, the probability that the weight will have to be applied to a continuous zone is also higher, rendering the use of vectorial decomposition with a previous memorization of the angular positions of the parts of the continuous radially-directed wheel disc a very laborious method.